Coextrusion process for overcoming the curtaining effect

ABSTRACT

A process for minimizing or eliminating the curtaining effect in the melt-lamination of thermoplastic materials is provided. The process includes subjecting a molten thermoplastic stream to an inverse resistance to flow.

This is a division of application Ser No. 604,995 filed Apr. 27, 1984now U.S. Pat. No. 4,533,308 issued 8.6.85 which is acontinuation-in-part of Ser. No. 600,961 filed Apr. 16, 1984 nowabandoned.

TECHNICAL FIELD

The present invention relates to the melt-lamination of thermoplasticmaterials. More specifically, this invention relates to overcoming thecurtaining effect, and in addition to providing for convergence ofmolten thermoplastic streams at substantially equal flow velocitieswithout external die adjustment.

BACKGROUND ART

The existence of the curtaining effect in an extruded thermoplasticlayer is a problem that has long needed a solution. FIG. 1 depicts a twolayer laminate of thermoplastic materials, in which the curtainingeffect is shown to be present in both layers. As explained in detailbelow, the convergence of molten thermoplastic streams at substantiallyequal flow velocities in an extrusion die is known. However, a problemis that external adjustment of the die is required. The presentinvention is concerned with overcoming these problems in a multimanifoldextrusion die.

As exemplified by U.S. Pat. No. 3,877,857 to Melead, multiple meltchamber extrusion dies are known. This type of die has two die halvesbetween which a center divider extends, and has in each die half anupstream and downstream melt or manifold chamber connected by acommunicating channel having a narrow cross-sectional area. A moltenstream flows from the downstream manifold chamber through a secondchannel having a narrow cross-sectional area, and then converges withanother molten stream to form a melt-laminate.

Also known, as illustrated by U.S. Pat. No. 3,694,119 to Scheibling, isan extrusion nozzle having a central tongue separating two flow passagesthat terminate at a discharge slot. In a special embodiment of theextrusion nozzle, the feed channels for the molten thermoplasticmaterials and/or the longitudinal slits connected to these channels areof such a construction that their cross-section is reduced toward thecenter of the channels and/or slits, and it is stated that this causesan improvement of the distribution of pressure in the material issuingfrom the extruders, so that very uniform layers are produced. In oneembodiment of the nozzle, a molten thermoplastic stream flows through apair of distribution channels joined by a narrow flow passage channel.

As exemplified by U.S. Pat. No. 4,344,907 to Herrington, a co-axialtubular extrusion die is known that has a flow restriction in the flowpath of the lower viscosity resin to increase the pressure drop of theresin as it passes through the die. Separating the two flow paths ofthis die is a divider having a wall that in part forms the flowrestriction.

Also known, as illustrated by FIG. 3 of my U.S. Pat. Nos. 4,152,387 and4,197,069, is a multimanifold coextrusion die having an adjustabledivider provided between any two of the flow channels thereof. Each flowchannel includes a back pressure cavity and a flow restriction channellocated between the back pressure cavity and the point of convergence ofthe flow channels. This coextrusion die provides for adjustment of flowrestriction channel width so as to cause the converging moltenthermoplastic streams to converge at substantially equal flowvelocities, by manual manipulation of the adjustable divider. As aresult, this die promotes laminar flow at the point of convergence.However, a drawback of this multimanifold extrusion die is that thelayers of a laminate produced thereby, exhibit the curtaining effect.Furthermore, even though the convergence of melt streams is effected atsubstantially equal flow velocities, external adjustment of the divideris required. Hence, there is a need for a multimanifold extrusion diethat minimizes or eliminates the curtaining effect, in addition toretaining the advances in the art provided by the prior art die of FIG.3 of U.S. Pat. Nos. 4,152,387 and 4,197,069. Such an improved die wouldbe especially remarkable if it were capable of overcoming the curtainingeffect for resins of varying viscosities and hence varying flow rates,merely by removal of and replacement of a component thereof with aninterchangeable component precisely configured for a specific resinviscosity. Moreover, there is a need for a multimanifold extrusion diethat in addition automatically provides for convergence of moltenthermoplastic streams at substantially equal flow velocities. Such a diewould make possible an improved process for melt-lamination ofthermoplastic materials.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide amultimanifold die that minimizes or eliminates the curtaining effect, inaddition to retaining the advances in the art provided by the prior artdie of FIG. 3 of U.S. Pat. Nos. 4,152,387 and 4,197,069.

It is a further object of the present invention to provide amultimanifold die of this type that is capable of overcoming thecurtaining effect for resins of varying viscosities, merely by removaland replacement of a component thereof with an interchangeable componentprecisely configured for a specific resin viscosity.

It is an even further object to provide a die of this type that inaddition provides for convergence of molten thermoplastic streams atsubstantially equal flow velocities without external adjustment.

It is an additional object to provide an improved process formelt-lamination of thermoplastic materials by which the curtainingeffect is minimized or eliminated.

Additional objects, advantages and novel features of the presentinvention are set forth in the description that follows, and in partwill become apparent to those skilled in the art upon examination of thefollowing description or may be learned by practice of the invention.

To achieve the foregoing objects and in accordance with the purpose ofthe present invention, as embodied and broadly described herein, thereis provided a multimanifold extrusion die for minimizing or eliminatingthe curtaining effect that moreover provides for convergence of moltenthermoplastic streams at substantially equal flow velocities withoutexternal adjustment. This die includes a first and a second flowchannel, which traverse the die and eventually converge at a point ofconvergence within the die, and further includes a free floating,pivoting divider means disposed between the flow channels.

Each of the flow channels includes a manifold chamber situated upstreamfrom the divider, and further includes in descending downstream order, apressure compensating restriction channel, an expansion chamber, and atapered flow restriction channel. The pressure compensating restrictionchannel is formed in part by a head portion of the divider. Theexpansion chamber has a cross-sectional area greater than that of anycross-section of the pressure compensating restriction channel, and isalso of greater cross-sectional area than the tapered flow restrictionchannel.

The manifold chamber has a longitudinal dimension of sufficientmagnitude that a molten thermoplastic stream exiting from the manifoldchamber is at a relatively greater pressure at a side-to-side midpointthan at the sides thereof. The pressure compensating restriction channelis of increasing cross-sectional area from the center to each endthereof so as to provide inverse resistance to flow whereby the moltenstream, which is flowing at relatively greater pressure at the midpointprior to flowing through the pressure compensating restriction channel,is caused to exit from this channel at substantially equal pressure fromside to side.

The expansion chamber and the tapered flow restriction channel each havea longitudinal dimension that maintains the molten stream at thesubstantially equal flow pressure.

A molten stream exits at the substantially equal flow pressure from thetapered flow restriction channel of each of the flow channels.Convergence of the molten streams forms a layered melt stream that flowsat substantially equal pressure from side to side.

The free floating divider automatically pivots in response to anydifference between the flow rates of the molten streams. As a result,convergence of the streams is at substantially equal flow velocities.

Also provided by the present invention is a process using amultimanifold extrusion die that minimizes or eliminates the curtainingeffect in the melt-lamination of thermoplastic materials. This processincludes subjecting a molten thermoplastic stream that is flowing at arelatively greater pressure at a side-to-side midpoint than at the sidesthereof, to an inverse resistance to flow whereby the molten stream iscaused to flow at substantially equal pressure from side to side. Thecross-sectional thickness of the molten stream is then expanded, whilethe substantially equal flow pressure is maintained. Next, thecross-sectional thickness of the expanded stream is thinned to a desiredcross-sectional dimension, while the substantially equal flow pressureis still maintained. The thinned molten stream is then converged with atleast one other thinned stream that is flowing at substantially equalpressure from side to side. The substantially equal flow pressure ismaintained for each stream of the resulting melt-laminate until themelt-laminate exits from the extrusion die.

In the drawing and in the detailed description of the invention thatfollows, I have shown and essentially described only a preferredembodiment of my invention, simply by way of illustration of the bestmode contemplated by me of carrying out my invention. As will berealized, my invention is capable of other and different embodiments,and its several details are capable of modification in various respects,all without departing from the invention. Accordingly, the drawing anddetailed description are to be regarded as illustrative in nature, andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWING

Reference is now made to the accompanying drawing which forms a part ofthe specification of the present invention, and which depicts apreferred embodiment of a multimanifold extrusion die in accordance withthe present invention.

FIG. 1 is a top view of a fragmentary portion of a two layer laminate,with a portion of an upper layer A removed, which shows the presence ofthe curtaining effect in each layer;

FIG. 2 is a view with an end plate removed of a preferred three layermultimanifold extrusion die in accordance with the present invention;

FIG. 3 is a cross-sectional view at mid-center of the die of FIG. 2,which shows a decreased cross-sectional area for each of the pressurecompensating restriction channels vis-a-vis the respectivecross-sectional areas as shown in FIG. 2;

FIG. 4 is a magnified fragmentary view of a pressure compensatingrestriction channel of the die of FIG. 3 taken from the direction shownby the line 4--4, this view illustrating the difference in thedimensions of this restriction channel depending upon whether athermoplastic material of high viscosity or low viscosity is passedthrough this channel; and

FIGS. 5 and 6 are magnified fragmentary views of a pressure compensatingrestriction channel, expansion chamber and tapered flow restrictionchannel of the die of FIG. 3, which show that the pressure compensatingrestriction channel is tapered regardless whether the tapered flowrestriction channel is at a maximum or minimum flow restriction.

DETAILED DESCRIPTION OF THE INVENTION

As explained above, the present invention is directed to an improvedmultimanifold extrusion die and to an improved process for themelt-lamination of thermoplastic materials. More particularly, thisinvention minimizes or eliminates the curtaining effect, a well knowndefect in the layers of a thermoplastic laminate. A longfelt need met bythe present invention is that it minimizes or eliminates the curtainingeffect, while at the same time it retains the advances in the artprovided by the prior art die of FIG. 3 of U.S. Pat. Nos. 4,152,387 and4,197,069. Moreover, this improved die does not require externaladjustment for convergence of molten thermoplastic streams atsubstantially equal flow velocities. Rather, it has a self-balancingfeature.

Beneficially, the present invention is useful in forming a laminatedfrom thermoplastic materials having similar or dissimilar flowproperties. Exemplary thermoplastic materials include, but are notlimited to, low- and high-density polyethylene, polypropylene,polycarbonates, polyamides, polyvinylchloride, polyvinylidene chloride,polystyrene, polyvinylacetate, polyacrylonitrile and copolymers thereof.

Referring to FIGS. 2 and 3, a preferred multimanifold extrusion die 10in accordance with the present invention is shown. Die 10 provides forthe passage of three thermo-plastic streams through flow channels 12, 14and 16, and for the convergence of these streams at a point ofconvergence 18 to form a three layer a melt-laminate. It will beunderstood that a die in accordance with the present invention away beused to form, for example, a two layer, five layer or seven layerlaminate, or may even be used to form a laminate having a greater numberof layers.

The melt-laminate is passed through an exit channel that consists of apreland channel 20, which is advantageously tapered, and a land channel22, which typically has parallel walls when viewed in cross-section. Themelt-laminate exits from the exit channel of die 10 at an opening 24.

Situated between flow channels 12 and 14 is a free floating, pivotingdivider or vane blade 26 and between flow channels 14 and 16 is a freefloating, pivoting divider or vane blade 28. Dividers 26 and 28 havehead portions 30 and 32, and point portions 34 and 36, as shown. At oneend of each divider is a round shaft (not shown), which pivots in abearing (also not shown). Each divider is permitted to pivot freely inresponse to any difference between the flow rates of the molten streamspassing through the adjacent flow channels, which are, for example,channels 12 and 14 in the case of divider 26.

Each flow channel includes a manifold chamber, shown as 46, 48 and 50; apressure compensating restriction channel, indicated as 52, 54 and 56;an expansion chamber, designated as 58, 60 and 62; and a flowrestriction channel, numbered as 64, 66 and 68.

For simplicity, the below description will be in terms of the featuresof one of these flow channels, but it will be understood to apply to theother two flow channels of die 10. Manifold chamber or manifold 46 islocated upstream from divider 26 and is a coat-hanger type manifold,that is, it has a cross-sectional area that diminishes from the centerto each end thereof. This characteristic of manifold 46 is revealed bycomparison of the cross-sectional area of manifold 46 in FIG. 3 with thecross-sectional area of manifold 46 in FIG. 2, which shows an end 70 ofmanifold 46. The manifold may be of constant or variable cross-sectiondepending upon, for example, individual thermoplastic materialrequirements. A manifold having a constant cross-section from end to endis known as a keyhole type manifold. The residence time of athermoplastic material is greater in a keyhole type manifold than in acoat-hanger type manifold. Accordingly, the latter type is preferredwhere, for example, it is advantageous for residence time to be as shortas possible due to, for purposes of illustration, thermal sensitivity.Also, the coat-hanger type may be preferred over the keyhole type forthe reason that a previously used thermoplastic material is more rapidlycleaned from this manifold, as a new thermoplastic material flowsthrough.

In the manifold, transverse flow of a molten stream occurs as a resultof which the stream is longitudinally distributed over the entire lengthof the manifold. The longitudinal dimension of a manifold ofmultimanifold die 10 is of considerable magnitude, that is, typically ofabout 10 to 60 inches or more, with the resulting effect being that astream exiting from the manifold is at a relatively greater pressure ata side-to-side midpoint than at the sides thereof.

The size of a manifold in terms of its cross-sectional area isdetermined by the required thermoplastic material throughput. Thus, withreference to FIG. 3, it can be seen that manifold 46 has a smallercross-sectional area than manifold 48 for the reason that a highermaterial throughput through manifold 48 is desired in die 10.

Pressure compensating restriction channel 52 is located downstream frommanifold 46 and is formed by a longitudinal wall 72 of head portion 30of divider 26 and an inner wall 74 of die 10. Channel 52 is especiallycharacterized by having increasing cross-sectional area from center 76thereof to each end of the channel, with an end 78 being shown in FIG.2. This feature of the channel is further illustrated in FIG. 4 in thecase of pressure compensating restriction channel 54, with arrows beingused in the FIG. to designate the width at the center of channel 54.Moreover, as can be further seen from FIG. 4, the dimensions of apressure compensating restriction channel are to be varied dependingupon whether a resin of relatively higher or lower viscosity (dottedlines in FIG. 4 represent the dimensions for the lower viscosity resin)is to be passed through the channel. The variable cross-sectional areaof channel 52 provides inverse resistance to flow to the thermoplasticstream that has exited from manifold 46 at a relatively greater pressureat a side-to-side midpoint than at the sides thereof. The precisecross-sectional dimension to be used for channel 52 is selected withregard to the viscosity of the particular resin to be passed throughthis channel so as to cause the molten stream to exit from channel 52 atsubstantially equal pressure from side to side. Hence, this channelsubjects the molten stream to an inverse resistance to flow in such away as to cause the stream to flow at substantially equal pressure fromside to side. I believe that this is a necessary aspect of minimizing oreliminating the curtaining effect in a die.

Referring to FIGS. 5 and 6, it can be seen that channel 52, when viewedin cross-section, is tapered in the direction of flow, regardless ofwhether downstream flow restriction channel 64 is at a minimum (FIG. 6)or maximum (FIG. 5) flow restriction, and that it provides relativelygreater flow restriction when channel 64 is at the maximum flowrestriction. Alternatively, channel 52, when viewed in cross-section,may be parallel when channel 64 is at the minimum flow restriction. Thenecessary configuration, from both a longitudinal and cross-sectionalstandpoint, is provided to channel 52 by longitudinal wall 72, withdivider 26 typically having been machined to form this wall.

The pressure compensating restriction channel feeds the moltenthermoplastic stream, which is now flowing at a pressure that issubstantially equal from side to side, to expansion chamber 58. Thischamber has a cross-sectional area that is greater than the area of anycross-section of channel 52. In fact, chamber 58, which is beneficiallyof substantially constant cross-sectional area from end to end, will betypically considerably greater in cross-sectional area than channel 52,as is illustrated in FIGS. 2 and 3. Hence, an important functionprovided by chamber 58 is that of expanding the thickness of the flowingmolten stream so that, for example, the stream can be subsequentlymetered by flow restriction channel 64 to a desired cross-sectionalthickness.

Expansion chamber 58 is formed by longitudinal wall 72 of anintermediate portion of divider 26 and inner wall 74 of die 10. Anessential characteristic of the chamber is that it has a longitudinaldimension that maintains the molten stream at substantially equalpressure from side to side. Thus, chamber 58 will characteristicallyhave the same longitudinal dimension as manifold chamber 46. It ispossible, however, that there could be a minor deviation in thelongitudinal dimension, provided that the substantially equal flowpressure effected by channel 52, is not adversely disturbed as themolten stream flows through expansion chamber 58.

From the preceding explanation concerning chamber 58, it can beunderstood that this chamber functions to expand the thickness of theflowing stream, while maintaining the stream at the substantially equalflow pressure. Moreover, it may be observed from FIGS. 2 and 3, thatwhereas the upper portion of chamber 58 is so configured as to providefor expansion of the stream thickness, the lower portion of the chamberis tapered, when viewed in cross-section, in the direction of flowrestriction channel 64, with which it merges to provide a channel forthe molten stream to flow to point of convergence 18.

Flow restriction channel 64 is also tapered, when viewed incross-section, in the direction of point of convergence 18. This channelis of smaller cross-sectional area than chamber 58, and thus serves tothin the cross-sectional thickness of the expanded molten stream.Channel 64, which is advantageously of substantially constantcross-sectional area from end to end, is formed by longitudinal wall 72of point portion 34 of divider 26 and inner wall 74 of die 10. Thecross-sectional dimension of channel 64 is precisely regulated bypivoting of vane blade 26 in response to any difference between the flowrates of the molten streams passing through flow channels 12 and 14. Thefree floating vane blade is self balancing: it provides for a relativelywider channel 64 than 66 in response to a relatively higher viscositymolten stream in channel 12 than 14, for example. Channel 64 has alongitudinal dimension that maintains the molten stream at thesubstantially equal flow pressure. Accordingly, this channel willtypically have substantially the same longitudinal dimension as manifoldchamber 46. Again, conceivably there could be a slight variation in thelongitudinal dimension, provided that the substantially equal flowpressure of the molten stream is not adversely disturbed as it passesthrough this channel.

From the above description of flow restriction channel 64, it will beunderstood that the molten stream exits from this channel atsubstantially equal flow pressure from side to side. Moreover, it willbe understood that channel 64 functions to thin the expanded moltenstream to a desired cross-sectional area, while maintaining the streamat the substantially equal flow pressure.

As a consequence of the location of channel 64 downstream from expansionchamber 58, a further function served by expansion chamber 58 is that ofa back pressure cavity. Similarly, the location of pressure compensatingrestriction channel 52 causes manifold chamber 46 to function as a backpressure cavity.

As might be expected, a difficulty that confronted me in inventing themultimanifold extrusion die of the present invention, was that ofeliminating or minimizing the curtaining effect, while at the same timeretaining the advances in the art provided by the multimanifold die ofFIG. 3 of my U.S. Pat. Nos. 4,152,387 and 4,197,069. Advantageously, thepresent invention achieves this objective.

Furthermore, the present invention concomitantly makes possible afurther improvement: it provides for convergence of molten thermoplasticstreams at substantially equal flow velocities without externaladjustment of the die. This additional advance in the art is a furtherbenefit of the configuration that I give to each flow channel for thepurpose of minimizing or eliminating the curtaining effect. Moreprecisely, the configuration of the manifold chamber, the pressurecompensating restriction channel, the expansion chamber and the taperedflow restriction channel enables the vanes to be free floating and pivotin response to a flow rate differential. In contrast, it is necessary inthe die of FIG. 3 for the vanes to be manually adjusted and locked intoposition, unless the flow rate differential will be insignificant orzero.

An additional advantage of my preferred die is that each divider isremovable and may be replaced with an interchangeable divider, in orderto provide die 10 with a pressure compensating restriction channelhaving a configuration precisely suited to the viscosity of a moltenresin to be passed through the channel. This avoids the necessity ofotherwise using a completely different die having a pressurecompensating restriction channel of the needed configuration. Thisadvantage will reduce cost in terms of both time and money.

As a side point with respect to the adaptor of my U.S. Pat. Nos.4,152,387 and 4,197,069, the layers of a laminated product made by useof the adaptor exhibit the curtaining effect even though a molten streamexerts equal pressure along the length of the elongated flow restrictionchannel thereof.

In operation, a molten thermoplastic stream enters flow channel 12, andpasses through manifold chamber 46, in which it is longitudinally spreadand out of which it flows at relatively greater pressure at aside-to-side midpoint than at the sides thereof. The molten stream nextpasses through pressure compensating restriction channel 52, whichsubjects the stream to an inverse resistance to flow as a result ofwhich the stream leaves channel 52 flowing at substantially equalpressure from side to side. The stream then passes through expansionchamber 58, in which the cross-sectional thickness of the stream isexpanded, while the substantially equal flow pressure is maintained. Theexpanded stream is subsequently passed through flow restriction channel64, in which the cross-sectional thickness is thinned to a desiredcross-sectional dimension. Next, the thinned molten stream is convergedat point of convergence 18 with two other molten streams each of whichis also flowing at substantially equal pressure from side to side. Atthe point of convergence, the molten streams have substantially equalvelocities. The resulting melt-laminate is passed through prelandchannel 20, then through land channel 22, and finally exits from die 10at opening 24.

In my process for minimizing or eliminating the curtaining effect in thea melt-lamination of resins, the essential steps are as follows. Amolten thermoplastic stream that is flowing at a relatively greaterpressure at a side-to-side midpoint than at the sides thereof, issubjected to an inverse resistance to flow whereby the molten stream iscaused to flow at substantially equal pressure from side to side. Thethickness of the stream is then expanded, while the substantially equalflow pressure is maintained. With this flow pressure continuing to bemaintained, the cross-sectional thickness of the expanded stream isthinned to a desired cross-sectional dimension. The thinned moltenstream is converged with at least one other thinned molten stream thatis flowing at substantially equal pressure from side to side to form amelt-laminate. The substantially equal flow pressure of each stream inthe melt-laminate is then maintained until the melt-laminate exits fromthe die. In a multimanifold die in accordance with the presentinvention, this last step is achieved by providing the exit channel witha longitudinal dimension that maintains each layer of the melt-laminateat the substantially equal flow pressure. Typically, the exit channelwill have the same longitudinal dimension as any of the manifoldchambers, which characteristically will all have the same longitudinaldimension.

In FIG. 2, I show each of the divider head portions provided with anadjusting device, shown as 38 and 40. As is clear from the abovedescription of my invention, these adjusting devices are not necessary,as the dividers are free floating and provide any required adjustment inthe relative widths of the adjacent flow channels in response to anyflow rate differential. Arrows 42 and 44 show that these adjustingdevices could be moved clockwise or counterclockwise.

Flow instability between the layered melt streams of the melt-laminatemay be reduced by locating point of convergence 18 as close as possibleto opening 24, that is, by making the exit channel as short as possible,for example, about 11/2 inches. Otherwise, the exit channel may be, forpurposes of illustration, about 4-6 inches.

In the preceding description of the present invention, there is shownand essentially described only one preferred embodiment of my invention,but as mentioned above, it is to be understood that the invention iscapable of changes or modifications within the scope of the inventiveconcept expressed herein. Several changes or modifications have beenbriefly mentioned for purposes of illustration.

I claim:
 1. A process for extruding thermoplastic materials, saidprocess comprising the steps of(a) passing a molten thermoplastic streamthat is flowing at a relatively greater pressure at a side-to-sidemidpoint than at the sides thereof, through a multimanifold dierestriction channel of increasing cross-sectional area from the centerthereof to each end thereof, so that the molten stream is subjected toan inverse resistance to flow and thereby caused to flow atsubstantially equal pressure from side to side; (b) while maintainingsaid substantially equal pressure, expanding the cross-sectionalthickness of the molten stream; (c) while continuing to maintain saidsubstantially equal pressure, reducing the cross-sectional thickness ofthe expanded stream; (d) converging the thickness-reduced molten streamwith at least one other thickness-reduced stream that is flowing atsubstantially equal pressure from side to side, to form a melt-laminate;and (e) maintaining said substantially equal flow pressure for eachstream of said melt-laminate, until said melt-laminate exits from saidmultimanifold die whereby the curtaining effect is minimized oreliminated in said melt-laminate.
 2. The process of claim 1, whereinthree molten streams are converged to form a three layer melt-laminate.3. The process of claim 1, wherein said thermoplastic materials areselected from the group consisting of low- and high-densitypolyethylene, polypropylene, polycarbonates, polyamides,polyvinylchloride, polyvinylidene chloride, polystyrene,polyvinylacetate, polyacrylonitrile and copolymers thereof.
 4. Theprocess of claim 1, wherein said thermoplastic materials include low- orhigh-density polyethylene or copolymers thereof.
 5. The process of claim1, wherein said thermoplastic materials include polypropylene ofcopolymers thereof.
 6. The process of claim 1, wherein saidthermoplastic materials include polyamides or copolymers thereof.
 7. Theprocess of claim 1, wherein said thermoplastic materials includepolystyrene or copolymers thereof.
 8. The process of claim 1, whereinsaid thermoplastic materials include polyvinylidene chloride.